The cell membrane is composed of a phospholipid bilayer containing proteins and carbohydrates. Choose ALL of the statements that accurately reflect the structure and its function.

1. Membrane proteins may be enzymes to catalyze metabolic reactions within the cell.

2. Membrane proteins may contain cholesterol to serve as identification tags for other cells.

3. Channel proteins allow a specific molecule to cross the cell membrane that is unable to cross the
phospholipid bilayer unassisted.

4. Receptor proteins bind to ligands, initiated cell signaling pathways.

5. Molecules that are large or nonpolar must use membrane proteins for transport